Most high capacity disk drives used in computer systems employ thin film magnetic media for storing binary encoded data. In a typical disk drive, a magnetic disk is rotated at a high speed about a central axis and the data are written and read by a magnetic head. In storing and retrieving data from the magnetic storage disk, the magnetic read/write head typically rides on a thin cushion of air as it moves along the data tracks along the surface of the rapidly spinning disk.
In many disk drives, the head rests on the surface of the disk when the drive is turned off. When the disk starts spinning, the head slides along its surface for some distance until the disk reaches a rotational speed at which the head becomes airborne. The reverse process takes place when the disk is brought to a stop. This sliding contact between the head and the disk may result in damage to the disk and therefore, to reduce wear and friction, a thin film of lubricant is normally applied to the disk surface.
Problems of stiction may also occur. If the disk topography is too smooth, the head will "weld" to the surface while the disk is at rest, and if the lubricant film is too thick, the surface energy of the film will "bond" the head to the disk surface.
It has been found that these problems are minimized if a texture of very fine grooves, separated by ridges, is created by abrasion on the surface of the disk. The grooves may act as reservoirs for the lubricant so that it can be replenished as it is worn off by contact between the head and the ridges and they overcome stiction by preventing the head from coming into contact with a continuous flat surface of the disk when it is at rest. The texture is normally formed in a hard layer of nickel-phosphorous material which overlays the relatively soft aluminum substrate of the disk, and the texture is perpetuated as additional thin film layers, including the magnetic layer, are deposited on the hard layer.
A large number of texture patterns are possible, ranging from one extreme, in which all of the grooves are concentric circles about the axis of the disk, to the other extreme, in which all of the grooves are oriented radially to the axis of the disk. The direction of the grooves is important in achieving optimum performance, since the grooves may affect the direction of easy magnetization and the magnitude of the coercive force.
For certain thin film magnetic alloy compositions, the anisotropy in magnetic characteristics which is caused by the grooves benefits performance, particularly for high density recording. For these compositions, circular grooves tend to yield higher coercivities in the circumferential direction. This is an advantage, as it is in the rotational direction of the head where the highest number of flux changes per inch are desired. Moreover, the lower coercivity in the radial direction minimizes "off track" noise. On the other hand, purely circular grooves may increase the extent of wear as the head slides on the ridges, while bit shift or phase margin defects may result if the grooves have too large a radial component. For these reasons, the present view is that a pattern which includes primarily circular grooves with a small radial component is desirable.
Whatever pattern of grooves is selected, it is important that the texture be extremely uniform. Surface asperities will cause glide problems and may cause failure in start-stop cycle tests.
Several types of texturing systems are known, generally classified as "fixed abrasive" or "slurry abrasive". Fixed abrasive processes typically use abrasives bonded to a mylar tape. The disk to be textured is clamped at its inside circumference, and rotated. The tape is supported by a cylindrical surface and pressed against the disk. While this system allows both sides of the disk to be textured simultaneously, it is subject to several disadvantages. First, relatively large clamping and rotational forces are applied, normally at the central aperture of the disk, because the abrasive tape is acting as a "brake" on the surface of the disk. These large forces may cause distortion of the disk, particularly with the thinner substrates now being introduced. Second, many abrasives are not available in tape form, and some abrasives and pad combinations that are available, such as diamond, are prohibitively expensive. Third, problems with texture uniformity, imbedding, asperities, etc. sometimes require the use of two-step tape processes. This increases the cost.
There are two types of abrasive slurry machines: units which employ a pad mounted on a rotating quill wheel, and units which are similar to the tape machine but use a cloth tape. In both types, a free abrasive slurry is sprayed onto the surface of the disk. These systems allow a wider choice of abrasives and tend to offer a less expensive texture process. Moreover, a large selection of pad materials is available for the rotating quill units. On the other hand, the rotating quill units cannot texture both sides of a disk simultaneously, are difficult to automate, and require frequent pad replacement. The cloth tape units require the disk to be firmly clamped, and the selection of cloth "tapes" is extremely limited. In any slurry machine, the abrasive particles in the slurry may settle or agglomerate into larger particles, particularly in the slurry supply lines.